In the field of ophthalmic aberrometry, wavefront aberration data provides a comprehensive description of human eye's optical properties, from which the refractive error as well as image quality metrics can be calculated.
Wavefront technology is based primarily on precise measurements of eye's wave aberration data using a device called wavefront sensor.
Numerous commercial wavefront sensors for the eye are available with most based on Shack-Hartmann wavefront sensor technology, as described in Liang et al. 94', “Objective measurement of wave aberrations of the human eye with the use of a Hartmann-Shack wave-front sensor,” J. Opt. Soc. Am. A, vol. 11, no. 7, p. 1949, July 1994.
FIG. 1 illustrates some principal elements of a basic configuration of the Shack-Hartmann wavefront sensor 10.
A measurement instrument employing the Shack-Hartmann wavefront sensor injects incoming wavefront light into a human eye which focuses on the retina of the eye and scatters back toward the measurement instrument.
The Shack-Hartmann wavefront sensor consist of an array of lenslets 11 in the plane conjugated to the pupil of the human eye and an optical detector 12 such as a charge-coupled device (CCD).
This incoming wavefront light is imaged onto the lenslets array 11 and produces a wavefront sensor image 13, 14 as an array of focus spots 13a, 14a formed on the focal plane of the optical detector 12 and recorded by the optical detector 12.
The locations of the focus light spots 13a, 14a are sensed and used to derive wavefront slope at each lenslet 11.
As the wave-front slope at each lenslet changes, each focus light spot 14a deviates relative to its reference location in the reference pattern 13 (i.e., the locations that result when a true plane wave is applied to the lenslet array) proportionally.
Wave-front aberrations data and wavefront reconstruction may be derived by calculating the centroid of each focus light spot 14a, and fitting the local slopes to a set of basis function such as Zernike polynomials.
The image quality is essential to ensure the accuracy of the optical measurements and the quality of the wavefront reconstruction.
Indeed, inaccurate focal light spot distributions, inaccurate focus light spot location data and/or inaccurate focus light spot intensity data will introduce errors into the estimate of the wave aberration data to the extent that it displaces the centroid computed for each focus light spot.
Wavefront reconstructed from the centroids of the inaccurate focus light spots provides an improper description of the optical aberrations of the tested optical system.
Wavefront reconstruction from conventional algorithms can be compromised and degraded by several artifacts that affect the wavefront sensing image data quality by incorporating spurious aberrations.
Conventional algorithms have proposed solutions to manage some artifacts such as the specular reflex from cornea.
However, others artifacts are underestimated.
For example, this is the case for the following artifact.
Conventional wavefront reconstruction algorithms assume that the wavefront sensing image is produced by light reflected from a single layer of fundus, which contradicts OCT (optical coherence tomography) results showing multiple layers contribute to the reflection.
More precisely, conventional wavefront reconstruction algorithms assume that the wavefront sensing image is produced by light reflected from a single point source which is located at a constant distance from the light-receiving layer in the tested eye.
In fact, the exact position of the point source is not mastered. Indeed, the retina is a rather thick reflector and multilayer and a probe beam is simultaneously reflected by multiple layers of the retina.
The focus light spots resulting from different reflective layers are laterally displaced and axially superimposed, which gives rise to spot elongation in the wavefront sensing image.
Therefore, conventional wavefront reconstruction algorithms lead to errors of unknown magnitude and wavefronts reconstructed will introduce spurious amounts of aberrations such as spherical aberration and defocus.
Thus, the management of noise and artifacts in the wavefront sensor data that would contaminate the measurements is a key issue.
There is a need to a method for determining wave-front aberration data to overcome the degrading effects of aberrations in image data and to respond to measurement accuracy and precision requirements.